It's a common scenario that happens to many individuals trying to gain muscle mass. Despite all their well-meaning efforts, in addition to the muscle they have successfully gained, they have also successfully gained a bit of a spare tire. Fat gain is definitely a consequence of trying to put on muscle mass because you are, after all, eating more calories than the body needs.
While you do require additional calories to build muscle, you can only assimilate so much muscle at one single time. Anything additional after that is going to end up, most likely, in the place you want it the least - usually around the waist for men and in the hips and thighs for women.
If you take a smart approach to your bulking program though, you can help minimize the fat gains that you see, while still being able to create a head-turning physique.
How To Maximize Your Anabolic Window
Training
If you've spent any time at all researching on the topic of weight lifting, you'll likely know that weight lifting in itself, accompanied by sufficient rest is an anabolic process. This basically means that after it's performed, the body is going to work at developing more muscle tissue rather than breaking it down.
Endurance cardiovascular exercise, on the other hand, is catabolic in nature - that is, the body will break tissues down. Since weight lifting is anabolic then, you want to do whatever you can to maximize all the cellular processes that are occurring, enhance recovery ability (so you can get back into the gym sooner, thus spending more time in an anabolic state), and improve nutrient partitioning.
Since your body is mostly going to shuttle any calories you feed it to the muscle cells around the workout, there is a lower chance that it's going to end up as that unwanted body fat you so dread.
Therefore, the first step when bulking is to get on a program that has you lifting heavy. If you want your muscles to grow, you d@mn well better give them a reason to grow. Unless you're challenging yourself, this simply is not going to happen. Remember, your body is thoroughly happy just hanging out as it currently is. While you may want change, it has other plans.
Rest & Recovery
After you've established a solid program (preferably a full body/3 X per week or upper/lower split), then you also must ensure enough rest.
Regardless of what you eat, if you don't give your body time to utilize these nutrients and grow back stronger between your sessions, you've just taken that anabolic process and turned it catabolic. This is not what you want. So repeated, ensure sufficient rest.
No matter how much you like training, do not make the gym your home away from home. Consistency is key, but setting up shop next to the free-weight section is not.
Nutrition
Then comes your dietary approach. This is what will most effectively produce a high anabolic period because you are going to load your muscles with carbohydrates immediately following each workout, trying to get as many as possible into those muscle cells.
Because of the nature of this approach, you are actually going to use a slightly higher surplus on weight-lifting days, as we'll be knocking it back on non. Usually you would go with about 250-500 extra calories per day, every day, in hopes of accomplishing about 1/2 to one pound gain of body tissue a week.
With this approach, we're going to ramp it up to between 750-1000 extra calories on days you are training. The majority of these calories should be placed before, potentially during, and after the workout.
You are to have a good sized pre-workout meal consisting of about 25-50 grams of protein and a good dose of carbs (exact dose will depend on how many total calories you're going to consume).
If your daily allotment for carbs is 300 grams for instance, go with about 75 grams pre-workout, 150 grams post-workout (note that this can be divided into two separate intakes - one immediately following the workout, and the other a short time after), and then leave the remaining 75 to have throughout the rest of the day along with more fat and protein based meals. Also be sure to include another 25-50 (or more if calories allow) grams of protein in the post-workout meal.
Fat should be avoided during this time and protein is usually always set at between 1-1.5 grams per pound of body weight per day so adjust your pre/post workout intakes accordingly.
The types of carbs should be both slow and fast acting as this will give you the initial insulin spike right after the workout and then lasting energy (and effective muscle glycogen re-synthesis) for the time period following.
As touched on above, the remainder of the day (and the portion before the workout if your session is in the later afternoon or evening) should be comprised of mostly fat and protein meals, still staying in that total calorie surplus for the day.
Minimizing Fat Gain
So, now that we've established what you're going to do on workout days to maximize muscle-building, now we'll talk about minimizing fat gain. The thing with the body is that it will adjust its metabolic rate to how much you are consuming.
Don't be misled though, if you go out and eat 10,000 calories, your metabolism is not going to magically adapt to this and start requiring this much from that point forward; there are limits. But, by shocking the metabolism with a different intake than it's used to, you force it to keep up with you, running faster at all times.
This can work to your benefit as far as limiting fat gain because if you drop calories down to just a few below maintenance on days you aren't working out, you can hopefully keep fat gain at bay, and potentially even lose fat (however this is the exception, not the norm). Most people cannot lose fat and build muscle at the same time.
That said, on days you aren't in the gym lifting, try and get there to do a small amount of cardio. Don't go crazy doing hours upon hours as this will limit your recovery and hinder your lifting sessions. Just get in, do 30-45 minutes two to three days a week at moderate intensity and call it a day. This will both help to create a slight deficit (necessary for fat loss), and help to increase blood flow to the muscles, improving recovery.
Also on these days, bring your calories down to about 100-200 below your required maintenance. This means you now have a zig-zag diet where you eat 750-1000 calories above maintenance on lifting days and 100-200 below maintenance on non-lifting days.
We don't want to bring down calories far too low on non-lifting days though, as that could hinder recovery (your body is still in recovery the next day) and also make gaining muscle quite slow as your weekly total calorie surplus would then be really quite low.
Most people will choose to reduce these calories from the carbohydrate portion of their diet, however you can also reduce it from fat if you prefer. Protein, however, should remain constant at that 1-1.5 gram/lb range.
Conclusion
So, next time you're looking to add some muscle, consider this option. It will take a little more time on your part calculating out your calories and different meals on the various days (rather than just keeping it at a constant intake every day), but it will be worth it when you find you're finished your muscle-gaining cycle and do not have to spend another three months just dieting off the fat you gained as well.